James Wittig
Interdisciplinary Materials Science, Vanderbilt University, 318 Keck Bld. Nashville TN, USA
A new class of austenitic steels stabilized with high-Mn contents (instead of Ni) exhibits exceptional mechanical properties, such as large energy absorption and high work-hardening rate, owing to secondary deformation mechanisms such as mechanical twinning-induced plasticity (TWIP) and martensitic transformation-induced plasticity (TRIP) favored for low stacking-fault energy (SFE) [1]. The interaction of dislocations with twin boundaries and martensite interfaces during mechanical deformation enhances the work hardening, i.e., a dynamic Hall-Petch effect, with total elongations exceeding 70% and ultimate tensile strengths in the GPa regime. The influence of the strain rate, temperature, and changes in SFE on the deformation mechanisms in high-Mn austenitic steels has been investigated using electron backscattered diffraction (EBSD), electron-channeling contrast imaging (ECCI), conventional bright-field/dark-field imaging (BF/DF), and aberration-corrected high-resolution transmission electron microscopy (HRTEM). The TWIP/TRIP secondary deformation mechanisms are related to the low SFE exhibited in these materials. Experimentally measured SFE from weak-beam-dark-field (WBDF) imaging provides the basis to understand how changes in SFE influence mechanical twinning versus transformation induced martensite [2-3]. However, adiabatic heating during deformation at high strain rates (100 -10,000 s-1) increases the SFE. Quantifying the twin or martensite density by EBSD/ECCI and BF-DF images allows for comparison of the secondary deformation at different SFE, strain rates, and total elongation, but to study the details of the deformation mechanisms requires imaging at atomic resolution using aberration corrected HRTEM. This presentation will correlate the mechanical properties of high-Mn austenitic steel with the deformation mechanisms and microstructure for strain rates from 10-4 to 103 s-1.
[1] O. Grassel, L. Kruger, G. Frommeyer, and L. W. Meyer, Int. J. Plasticity,16(2000) p.1391
[2] D T Pierce, JA Jiménez, J Bentley, D Raabe, C Oskay and JE Wittig, Acta Mater 68(2014)238-53
[3] D T Pierce, J A Jiménez, J Bentley, D Raabe and J E Wittig, Acta Mater 100(2015)178-90
